A method for performing fast base station switching in an mobile station capable of using multiple frequency assignments in a wireless communication system are provided, in which initial network entry to a base station managing at least two frequency assignments is performed in one of the at least two frequency assignments, overlay mode support information and FA information about the at least two frequency assignments are acquired by the initial network entry, the overlay mode support information indicating whether the base station supports multiple frequency assignments, the base station is notified that the mobile station will operate in an overlay mode using multiple frequency assignments based on the acquired information, and signals are transmitted and received to and from the base station in the at least two frequency assignments.